The U.S. reserve of coal is about 3 trillion tons. Although the most abundant (80%) fossil fuel in America is coal, the U.S. consumption pattern is quite a reversal of form in terms of utilization, with coal representing only 17%, oil and gas about 78%.
The demand for all fossil fuels combined is expected to double by the year 2,000, even with increasing the use of nuclear power. While the domestic supply of crude oil and natural gas is not likely to keep pace with the energy demand, coal can play an important role in filling such a gap and thus reduce the requirements for imported supplies of oil and gas.
Coal, the fossilized plant life of prehistoric times, contains various amounts of sulfur due to the nature of its origin. Under most existing commercial technology, the generation of electricity from coal poses environmental problems because of sulfur oxides and particulate emissions. Since most of the coals in this country, particularly the Eastern and Midwestern coals, have high sulfur content (&gt;2%) there is a need for an economical process of converting high sulfur 2% coals to clean fuel (&lt;1.2 lbs. of SO.sub.2 emission per million B.t.u. by EPA standard) in order to utilize coal as a source of energy without causing serious air pollution. So the need for converting massive coal reserves to clean-burning solid fuel, liquid fuel and pipeline quality gas is self evident. If the vast coal reserve is converted to clean fuel, it can supply most of the energy needs of the United States for the next three centuries.
At the present time, about one-half of the electric power in the United States is generated from natural gas and petroleum; most of the other half is from coal. If the coal is converted to clean fuel for electric utilities, petroleum and natural gas would be released for other essential uses, especially as a starting material for the synthetic rubber and plastics industry.
Sulfur in coal occurs in two types, generally in approximately equal amounts of inorganic sulfur primarily as pyrites with minor amounts of sulfates and of organic sulfur in the forms of thiophene, sulfide, disulfide and mercaptan chemically bound in the organic structure of coal.
The sulfur oxides in the combustion gases of coal can be removed by stack gas scrubbing methods but those are expensive processes and produce large amounts of sludge. Hydrodesulfurization processes which remove sulfur from the fuel before combustion are effective. They are used extensively in petroleum desulfurization and many coal conversion processes under development. However, they are also expensive due to the cost of hydrogen and severe operating conditions required.
Physical separation methods can only remove the inorganic sulfur. Other desulfurization schemes under investigation such as TRW Meyers' process and Battelle Hydrothermal Coal Process are either primarily for inorganic sulfur removal or are operated at high temperature and pressure resulting in high process cost and in the physical disintegration of the coal.
A promising new process utilizing chlorine for removing organic and inorganic sulfur is described in U.S. Pat. No. 4,081,250. The three-stage process includes an initial room temperature chlorine treatment of coal slurry suspended in solvent/water media. After chlorinolysis a batch hydrolysis and solvent recovery is carried out. Finally, dechlorination at 300.degree. C. to 450.degree. C. yields a desulfurized coal product. This process requires use of a chlorine resistant solvent such as methyl chloroform which is recovered by steam distillation.
Ser. No. 156,790 filed May 12, 1980, demonstrates that an aqueous medium is an equally effective, if not superior, medium for the chlorinolysis-desulfurization reaction, and Ser. No. 250,646 filed Apr. 3, 1981, discloses increased sulfur removal from coal by selection of a solvent having a solubility parameter matching peaks within the solubility parameter spectrum of coal.
However, even though the chlorinolysis process has proven to be an effective process for pre-combustion reduction of sulfur by about 60 to 70% of original sulfur content, only 20 to 30% of organic sulfur is removed. The treated coal contains significant amounts of chlorine. The chlorinized-desulfurized coal process will not meet new source performance standards for sulfur emissions of a maximum allowable emission of 1.2 pounds of SO.sub.2 per 10.sup.6 B.t.u. or 0.7 wt.% sulfur/12,000 B.t.u./lb. coal for all coals.
The organic sulfur remaining after chlorinolysis is extremely difficult to remove, especially without excessive loss of heating value and without expenditure of significant amounts of energy. Additional desulfurization is required. An investigation of conducting additional desulfurization during dechlorination by raising the temperature from 400.degree. C. to 700.degree. C. showed some additional increase in desulfurization but not substantially above that normally found at 400.degree. C. The addition of oxidizing agents such as NO.sub.2, SO.sub.2 of O.sub.2 to the inert nitrogen atmosphere in the dechlorination stage also showed no improvement in desulfurization.